Antigen-activated lymphocytes, or transformed lymphocytes in leukemias, must double their size and contents (termed cell growth) before they can divide into equal sized daughter cells. Despite the importance of cell growth in normal cell proliferation and cancer, the molecular events that control cell growth in dividing lymphocytes or other mammalian cells remain an enigma. Our preliminary studies in mice suggest that the Mad family of basic helix-loop-helix transcription factors (Mad1, Mxi1, Mad3, Mad4), considered to be antagonists of the Myc oncoprotein, inhibit T cell proliferation and development in part by inhibiting cell growth. Furthermore, human mad1 and mxi1 genes each localize to separate chromosome regions associated with lymphocytic leukemias, Hodgkin's disease, and prostatic carcinomas suggesting the importance of mad genes in lymphocyte biology and cancer. The broad objective of this proposal is to determine the normal roles and mechanism of action of Mad family members in the development and expansion of T Iymphocytes Specifically, we intend to: (1) Test the hypothesis that Mad family members modulate the maturation of T lymphocytes. We will examine the role(s) of Mad family members in T lymphocyte development by employing targeted deletion, and transgenic overexpression, of selected Mad family members in mice. (2) Test the hypothesis that Mad family members control the proliferation and cell growth (cell size, protein synthesis) of T Iymphocytes during T cell activation. We will examine the functional consequences of Mad overexpression or loss on cell division, cell size, RNA processing, and protein synthesis in primary lymphocytes immediately following activation. (3) Test the hypothesis that Mad directly binds and modulates the expression of essential genes involved in cell growth control. We will determine if Mad1 inhibits the expression of several essential genes involved in cell growth control. We will then use chromatin immunoprecipitation assays to determine if the regulatory regions of these genes are directly bound by Mad proteins. Together, these aims will test the overall hypothesis that Mad-Max complexes normally modulate lymphocyte proliferation and development in part by controlling the expression of growth-regulating genes. Results of these studies will identify target genes that could be manipulated to regulate the balance between Myc and Mad in order to inhibit lymphocyte proliferation in lymphomas or autoimmune disease, or to enhance clonal expansion of antigen-specific lymphocytes in a primary immune response, or following bone marrow transplantation.